The use of liquid crystal materials in matrix displays has received considerable attention. In general, the use of alternating current is preferred because of the fact that there is no net transfer of charge so that oxidation and reduction reactions are avoided. U.S. Pat. No. 3,575,492 discloses the use of alternating current in combination with nematic liquid crystal elements for increasing the speed of erasure of the display, a frequency in the range between 2 and 20 KHz being cited. U.S. Pat. No. 3,740,717 describes multiplexed operation of a plurality of liquid crystal numeric indicators in which a plurality of alternating exciting voltages are used. An operating principle based on change in the sign of the dielectric anisotropy of a liquid crystal material with change in frequency is described by B. Dargent et al in a paper "Twisted Nematic Flat-Panel Display" published in the April, 1977 issue of the Society For Information Display International Symposium, Digest of Technical Papers. The liquid crystal material used was ZLI 518 manufactured by Merck. A low frequency signal was used to turn the display on and a high frequency signal was used to turn it off.
While these papers disclose the use of more than one frequency in a matrix display as well as a knowledge and means of utilizing the change in dielectric anisotropy of the liquid crystal display material with frequency, nevertheless, these papers do not teach a means of coping with the problem of presenting a display in which the number of lines is large, such displays commonly suffering from low contrast. This problem is not solved by the multiplex driving method now being used, said method being termed the generalized AC amplitude selective multiplexing method. As is evident then, the improvement over the conventional method of matrix-addressing is strongly to be desired.